1. Field of the Invention
This invention relates generally to hybrid electric vehicles (HEV""s), and more particularly to a control strategy for a hybrid electric vehicle of the type in which a dynamoelectric machine, such as a combination starter/alternator, interacts with a combustion engine that drives road-engaging wheels through a drivetrain.
2. Background Information
In one type of hybrid electric vehicle (HEV), a combustion engine is coupled through a drivetrain to road-engaging wheels, and a dynamoelectric machine, such as a combination starter/alternator, is arranged to interact with the engine to modify the torque output of the engine. Such a starter/alternator may be structurally integrated with a combustion engine. Although the drivetrain may be considered the primary load on the engine, various accessory loads, such as a power steering pump and a refrigerant compressor for example, may at times contribute to engine load.
During vehicle operation, the dynamoelectric machine may develop torque that is additive to the engine torque during certain conditions, thereby augmenting the torque output of the engine, while during other conditions, it may instead impose a torque load on the engine, diminishing the magnitude of torque that can be delivered by the engine. In other words, when it is adding to the engine torque, the dynamoelectric machine functions as an electric motor, and when it is subtracting from the engine torque, it functions as an alternator, or electric generator, that is being driven by the engine.
When functioning as an alternator powered by the engine, the rotating electric machine generates electricity that can be used for any appropriate purpose, such as charging an on-board energy storage means, an example of which is a storage battery or battery bank. When functioning as a motor, the rotating electric machine may draw electric current from the energy storage means to supplement the torque output of the engine.
From the foregoing brief and somewhat general description, it can be understood that various possible modes in which a rotating electric machine can interact with a combustion engine require a sophisticated control system and control strategy. The availability of high-speed electronic devices, such as processors, enables sophisticated control strategies to be implemented in real time.
Torque produced by an internal combustion engine contains an average value and typically a large ripple value. The ripple is due to a combination of interactive factors including combustion pressure pulses and kinematics of the engine slider crank mechanism. It is known to utilize various mechanical devices, such as flywheels and dampers for example, to attenuate ripple. The inclusion of such devices adds weight to an engine and increases cost. It is therefore desirable if ripple could be effectively attenuated in other ways, such as by an effective strategy for control of the interaction of a dynamoelectric machine with a combustion engine crankshaft.
An observer is a computational or numerical process that may be implemented in a digital microprocessor or in a digital signal processor. The observer acts on inputs supplied to it from various sources, such as sensors, to produce an estimate, or observation, of a variable of interest that, for any of various reasons such as cost or convenience of access, may be poorly suited for direct measurement. As such, observers are useful in multi-variable control systems as alternatives to direct measurement of at least some of the variables of interest. A closed-loop observer in which an observer feedback signal is compared to a measured quantity in order to force the other estimated quantities to converge to their actual, correct values, may provide a higher degree of performance than one that is not closed-loop.
Torque of a rotating shaft is one variable that may be considered relatively difficult to accurately measure, even in laboratory conditions. Hence it is believed to be well suited for estimation by an observer. Moreover, torque observation may eliminate the need for a devoted torque sensor, thereby improving robustness of a multi-variable system.
In an automotive vehicle, observation of the torque output of an internal combustion engine crankshaft would be useful in the performance of various control functions related to vehicle powertrain operation, including for example engine control, transmission shift control, combustion diagnostics, and dynamometer testing. In a hybrid electric vehicle that has a starter/alternator associated with the engine, the electric machine can develop torque that can be independently controlled to interact with engine crankshaft torque in beneficial ways including: reduction of the ripple content of crankshaft torque, boosting transient crankshaft torque during vehicle launch and acceleration, recapturing breaking energy during vehicle decelerations, and retarding or boosting crankshaft torque for faster engine speed slewing during controlled shifting. Accurate observation of crankshaft torque in real time would be advantageous in controlling starter/alternator torque so as to secure its desired interaction with crankshaft torque in real time. Active control of the electric machine may enable it to function in a manner equivalent to a mechanical flywheel, and hence a starter/alternator that is controlled in this manner may sometimes be referred to as an active flywheel. An active flywheel may function to produce a zero mean value, destructively interfering torque that, when superimposed on the crankshaft torque, tends to reduce or eliminate ripple in the crankshaft torque caused by periodic combustion events in the engine cylinders.
In order to command the starter/alternator to produce correct torque for smoothing the ripple, precise instantaneous measurement of crankshaft torque during combustion events is needed. A crankshaft torque observer possessing this capability could be made an integral part of an overall fuel economy strategy for lowering engine idle speed while improving NVH (noise, vibration, harshness).
The co-pending patent application referenced above discloses a crankshaft torque observer that is useful in practice of the present invention.
The present invention relates to improvements in HEV control strategy implementation, particularly in an HEV of the type described above where a dynamoelectric machine is arranged for interaction with a combustion engine so as to function, with respect to the engine, at times as a power source, i.e. as an electric motor, for adding torque into the drivetrain, and at times as a power sink, i.e. as an alternator, for replenishing the charge of an on-board storage battery. This type of HEV is representative of what is sometimes referred to as a low storage requirement HEV. Although a starter-alternator that has a point of interaction with the engine at the engine crankshaft is believed to be a configuration that provides most accurate attenuation of ripple torque, generic principles of the invention contemplate both points of interaction that are more remote and devices other than starter-alternators.
In accomplishing the best control strategy, it is believed that 1) the phase of the cancellation torque should align as closely as possible to the phase of the ripple torque, and 2) the cancellation torque should have zero average value so as to neither buck nor boost the average engine torque driving the load. Principles of the present invention: employ the referenced crankshaft torque observer to provide feedforward and state estimates for use in a state feedback controller; account for the effect of the dynamoelectric machine, i.e. starter-alternator; and extract substantially only the ripple component of observed crankshaft torque and use it as a principal component of an electric torque command.
One generic aspect of the present invention relates to a hybrid electric vehicle comprising a combustion engine for propelling the vehicle via a drivetrain of the vehicle and comprising a slider-crank mechanism including a rotating crankshaft that delivers output torque to a load and a dynamoelectric machine arranged for interaction with the combustion engine to modify the output torque delivered by the crankshaft.
A control system comprising a processor develops data controlling the combustion engine and the dynamoelectric machine and includes an observer for developing observed data that comprises estimated engine speed, estimated crankangle, and estimated engine output torque represented as an estimated average value torque component on which is superimposed an estimated alternating polarity ripple torque component whose mean value over a time interval of interest is substantially zero.
The observer comprises a combustion process model developing modeled pressure estimates of combustion chamber pressures in engine cylinders according to data that influences combustion chamber pressures, a kinematics model relating reciprocal motion of pistons in the engine cylinders to engine crankshaft rotation, an engine friction model relating running friction of the engine to engine crankshaft rotation, an observer closed loop controller to force convergence of estimated crankangle to measured crankangle, a filter model developing the estimated alternating polarity ripple torque component from the estimated engine output torque, a dynamoelectric machine model modeling the dynamoelectric machine, an engine load model modeling load on the engine crankshaft, and a moment of inertia model modeling moment of inertia of a slider-crank mechanism of the engine as a function of crankangle.
The processor operates to process data through the combustion process model to develop the modeled pressure estimates, to process estimated crankangle and the modeled pressure estimates through the kinematics model to develop estimated positive torque contribution due to combustion processes, to process the modeled pressure estimates, the estimated crankangle, and the estimated engine speed through the engine friction model to develop estimated torque loss due to engine running friction, and to process the estimated positive torque contribution due to combustion processes and the estimated torque loss due to engine running friction to develop the estimated engine torque output.
The processor further operates to process the measured crankangle and the estimated crankangle through the observer controller to develop crankangle convergence data, to process the estimated engine torque output and the crankangle convergence data through the filter model to develop the estimated alternating polarity ripple torque component, and to process the estimated alternating polarity ripple torque component through the dynamoelectric machine model to develop estimated dynamoelectric machine torque output, to subtract the estimated dynamoelectric machine torque output from the estimated engine torque output and process that difference and the convergence data through the moment of inertia model to develop the estimated crankangle and the estimated engine speed.
The processor further operates to process the estimated crankangle, the estimated engine speed, and the estimated alternating polarity ripple torque component to develop dynamoelectric machine torque data representing torque that the dynamoelectric machine is commanded to deliver for substantially canceling, from output torque delivered by the crankshaft, engine-induced ripple torque that has substantially zero mean value over a time interval of interest, resulting in crankshaft torque that has substantially zero mean value RMS dynamic content over a time interval of interest.
Another generic aspect relates to a control system for a powerplant that comprises a combustion engine having a slider-crank mechanism including a rotating crankshaft that delivers output torque and a dynamoelectric machine arranged for interaction with the combustion engine to modify the output torque delivered by the crankshaft.
The control system comprises a processor for developing data for controlling the combustion engine and the dynamoelectric machine, including an observer for developing observed data that comprises estimated engine speed, estimated crankangle, and estimated engine output torque represented as an estimated average value torque component on which is superimposed an estimated alternating polarity ripple torque component whose mean value over a time interval of interest is substantially zero.
The observer comprises a combustion process model developing modeled pressure estimates of combustion chamber pressures in engine cylinders according to data that influences combustion chamber pressures, a kinematics model relating reciprocal motion of pistons in the engine cylinders to engine crankshaft rotation, an engine friction model relating running friction of the engine to engine crankshaft rotation, an observer closed loop controller to force convergence of estimated crankangle to measured crankangle, a filter model developing the estimated alternating polarity ripple torque component from the estimated engine output torque, a dynamoelectric machine model modeling the dynamoelectric machine, an engine load model modeling load on the engine crankshaft, and a moment of inertia model modeling moment of inertia of a slider-crank mechanism of the engine as a function of crankangle.
The processor operates to process data through the combustion process model to develop the modeled pressure estimates, to process estimated crankangle and the modeled pressure estimates through the kinematics model to develop estimated positive torque contribution due to combustion processes, to process the modeled pressure estimates, the estimated crankangle, and the estimated engine speed through the engine friction model to develop estimated torque loss due to engine running friction, and to process the estimated positive torque contribution due to combustion processes and the estimated torque loss due to engine running friction to develop the estimated engine torque output.
The processor further operates to process the measured crankangle and the estimated crankangle through the observer closed loop controller to develop crankangle convergence data, to process the estimated engine torque output and the crankangle convergence data through the filter model to develop the estimated alternating polarity ripple torque component, and to process the alternating polarity ripple torque component through the dynamoelectric machine model to develop estimated dynamoelectric machine torque output, to subtract the estimated dynamoelectric machine torque output from the estimated engine torque output and process that difference and the convergence data through the moment of inertia model to develop the estimated crankangle and the estimated engine speed.
The processor further operates to process the estimated crankangle, the estimated engine speed, and the estimated alternating polarity ripple torque component to develop dynamoelectric machine torque data representing torque that the dynamoelectric machine is commanded to deliver for substantially canceling, from output torque delivered by the crankshaft, engine-induced ripple torque that has substantially zero mean value over a time interval of interest, resulting in crankshaft torque that has substantially zero mean value RMS dynamic content over a time interval of interest.
A further aspect of the invention relates to a method of controlling a dynamoelectric machine to substantially cancel substantially zero mean value ripple torque induced in a crankshaft of a combustion engine by combustion events that are effective on the crankshaft through a slider-crank mechanism.
The method comprises processing data through an observer to develop observed data that comprises estimated engine speed, estimated crankangle, and estimated engine output torque represented as an estimated average value torque component on which is superimposed an estimated alternating polarity ripple torque component whose mean value over a time interval of interest is substantially zero.
The method further comprises: processing data through a combustion process model in the observer to develop modeled pressure estimates of combustion chamber pressures in engine cylinders according to data that influences combustion chamber pressures, processing estimated crankangle and the modeled pressure estimates through a kinematics model in the observer relating reciprocal motion of pistons in the engine cylinders to engine crankshaft rotation to develop estimated positive torque contribution due to combustion processes; processing the modeled pressure estimates, the estimated crankangle, and estimated engine speed through an engine friction model in the observer relating running friction of the engine to engine crankshaft rotation to develop estimated torque loss due to engine running friction; processing, in the observer, the estimated positive torque contribution due to combustion processes and the estimated torque loss due to engine running friction to develop indicted engine torque output; processing the measured crankangle and the estimated crankangle through an observer closed loop controller forcing convergence of estimated crankangle to measured crankangle; processing the estimated engine torque output and the crankangle convergence data through a filter model in the observer to develop the estimated alternating polarity ripple torque component from the estimated engine output torque; processing the estimated alternating polarity ripple torque component through a dynamoelectric machine model in the observer modeling the dynamoelectric machine to develop estimated dynamoelectric machine torque output; and subtracting the estimated dynamoelectric machine torque output from the estimated engine torque output and processing that difference and the convergence data through an engine load model in the observer modeling load on the engine crankshaft and a moment of inertia model in the observer modeling moment of inertia of the slider-crank mechanism as a function of crankangle to develop the estimated crankangle and the estimated engine speed.
The method further comprises processing the estimated crankangle, the estimated engine speed, and the estimated alternating polarity ripple torque component to develop dynamoelectric machine torque data representing torque that the dynamoelectric machine is commanded to deliver for substantially canceling, from output torque delivered by the crankshaft, engine-induced ripple torque that has substantially zero mean value over a time interval of interest, resulting in crankshaft torque that has substantially zero mean value RMS dynamic content over a time interval of interest.
Further inventive aspects will be disclosed and perceived from a reading of the ensuing description and claims with reference to the accompanying drawing.